DummyStepperMotor: Report interpolated angle while moving

[alexdewar]

(This is super-low priority seeing as DummyStepperMotor is purely a developer convenience, but I'm making an issue for it anyway so we can track it.)

Currently while a DummyStepperMotor is moving, it continues to report its old position until the move completes. It would be useful to get an intermediate angle instead (e.g. computed via linear interpolation) so that we can see it updating in the GUI etc. (#748). It's worth noting that the DummyStepperMotor always takes a fixed amount of time to move to any position, so it might also be more realistic to instead have a fixed angular velocity and compute the move duration from this.